This section provides background information related generally to the present disclosure which is not necessarily prior art.
Due to continuing consumer demand for four-wheel drive vehicles, a plethora of different power transfer systems are currently utilized for directing power (i.e., drive torque) to all four wheels of the vehicle. For example, in many “part-time” power transfer systems, a transfer case is installed between the front and rear drivelines and is normally operable in a two-wheel drive mode for only delivering drive torque to the rear wheels. However, when a four-wheel drive mode is desired, a mode shift mechanism is selectively actuated by the vehicle operator for directly coupling the front wheels to the rear wheels for establishing a part-time or locked four-wheel drive mode.
A significant number of the transfer cases are also equipped with a gear reduction unit and a range shift mechanism operable for permitting the vehicle operator to choose between high-range and low-range drive modes. In some instances, the vehicle must be stopped before the transfer case can be shifted between its high-range and low-range drive modes. Unfortunately, the need to stop the vehicle prior to shifting between the high-range and low-range drive modes is inconvenient, particularly upon encountering road conditions or surface terrains where continuation of the vehicle's rolling momentum would assist in overcoming the conditions encountered. To alleviate this inconvenience, some two-speed transfer cases are equipped with a synchronized range shift mechanism from permitting “on-the-move” shifting between the high and low ranges.
It is also known to use “on-demand” power transfer systems for automatically directing power to the front wheels, without any input or action on the part of the vehicle operator, when traction is lost at the rear wheels. Typically, this on-demand feature is incorporated into the transfer case by replacing the mode shift mechanism with a multi-plate clutch assembly and a power-operated clutch actuator that are interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the clutch assembly is maintained in a non-actuated condition such that drive torque is only delivered to the rear wheels. However, when the sensors detect a low traction condition at the rear wheels, the clutch assembly is automatically actuated to also deliver drive torque to the front wheels. The amount of drive torque transferred through the clutch assembly to the front wheels can be varied as a function of specific vehicle dynamics and operational characteristics detected by the sensors.
Conventional transfer cases are typically engineered for use in regular-duty trucks and passenger-carrying SUVs. Transfer cases are also used in heavy-duty vehicular applications, with such being commensurately sized to accommodate the higher load and torque requirements. However, the recent development of vehicles with lower numerical axle ratios and increased transmission output torques has mandated development of new transfer cases for heavy-duty vehicle application that are capable of accommodating the increased torque requirements. These so-called heavy-duty transfer cases but also typically requiring much larger and heavier components. The lower volumes associated with such heavy-duty transfer cases result in detrimental effects to their overall manufacturing cost and the vehicle's fuel economy and overall weight.
Accordingly, a recognized need exists to develop two-speed heavy duty transfer cases that address and overcome these and other disadvantages associated with conventional transfer cases.